It is currently known that photodynamic therapy (PDT) using hematoporphyrin and its derivatives (HpD) requires red light at the wavelength range of around 630 nm. This corresponds to one of the broad absorption maxima of HpD. For FDT applications, red laser outputs from argon ion pumped dye lasers (Forbes et. al., 1980), gold vapour lasers (Forbes et. al., 1984) and helium-neon lasers (Perria et. al., 1980) have been reported. Dye lasers can be tuned to output in the red from 620 to 630 nm whereas the gold vapour laser has an output at 628 nm and the helium-neon laser at 632.8 nm. All three lasers can deliver red laser light equally effectively to an optical fiber for photodynamic therapy. FIG. 1 shows how a simple lens is used to focus a laser beam into an optical glass fiber (typically from 400 to 1000 um diameter) for delivery of red laser light for PDT. Laser output power of a few hundred milliwatts (at least 600 mW as quoted J. A. S. Carruth and A. L. McKenzie, 1985 ) are necessary. Both the argon ion pumped dye laser and the gold vapour laser can deliver these powers. However, both types of lasers are technically difficult to operate. High electrical power consumption (usually from a 3-phase supply), water cooling, long start up time and elaborate maintenance procedures are needed.
On the other hand, helium-neon lasers are relatively simple lasers to use and can be operated from the usual domestic single phase supply. Helium-neon lasers are also known to be reliable with long operating life. However, presently available commercial helium-neon lasers can give at most 100 mW of stable output power. A single helium-neon laser has therefore too low optical power for practical photodynamic therapy. The possibility of folding a series of helium-neon lasers, either end-on or in folded geometry, has not been reported to be successful due to the difficulties in the alignment of such a long cavity laser due to the small bore size normally required of helium-neon laser tubes. The possibility of using a parallel array of helium-neon lasers, effectively coupled together, has not been conceived as a possible alternative to the dye laser or gold vapour laser systems for photodynamic therapy.